Conventional power steering devices for automobiles currently used are mainly oil-hydraulic types. These types of power steering devices have an oil pump as a hydraulic power unit, an oil-hydraulic actuator or converting oil pressure from the oil pump into mechanical strength to generate steering force, a control unit controlling operation of steering by opening or closing the passage between the oil pump and the actuator, and a number of additional components to ensure operational accuracy.
Therefore, these power steering devices require comparatively complicated mechanical structures, a large mounting space, an increase in weight, and high production costs. Furthermore, a regular replenishment of oil is required in order to maintain its best performance.
Consequently, various kinds of electrical power steering devices are known which replace the oil pump and the actuator by a steering motor. Japanese Patent Publication No. 83-202803 and No. 84-70257 disclose typical examples of these. These power steering devices generally have a steering motor connected to the steering link or to the steering shaft, a torque sensor detecting the torque of the steering wheel, a speed sensor detecting the running speed of a car, and a control unit changing the torque of the steering motor in accordance with the output from each sensor.
However, most of these electrical power steering devices have some fatal disadvantages. First of all, they have a high probability of malfunctioning such as, for example, unintended steering contrary to the driver's intentions, mostly due to the variation of voltage on the circuit, which is generated from the power source and/or the torque sensor. Secondly, they have a high probability of damage to the circuit due to the resistances used for changing the torque of the steering motor, and in case .of such damage, steering capability may be lost, bringing about extreme danger. Thirdly, they have structural difficulty in returning the steering wheel and the steering shaft to the straight direction when steering in a different direction is completed, because the steering motor rotates together with the steering wheel. Otherwise an additional circuit should be provided for this purpose.
Due to the drawbacks still unsolved, the electrical power steering devices have not been widely used in spite of their simple structure and light weight.
The inventor has disclosed an electrical power steering device in U.S. patent application Ser. No. 08/097,709 and 8/117,239. This electrical power steering device is believed to succeed in solving most of the problems described in the above, but a delay in steering response time occurred due to inertia rotation of the steering motor.
Accordingly, the inventor has studied how to acquire an immediate steering response in the electrical power steering device, and as a results, has found that this immediate steering response can be obtained by making the steering motor rotate in one direction at all times, and by changing the structure of transmission between the steering motor and the steering shaft.